Various metals are used in many useful industrial processes. For example, silver is used in many image-forming industrial processes, such as photography, thermography, and photothermography. Such processes, however, result in waste solutions containing metal ions at levels that may be undesirable to the environment. In addition, many countries now have laws that control the levels of certain metal ions that can be released into the environment. Since commercial disposal of large volumes of untreated waste solutions can be costly, there has been a concerted effort to treat the waste solutions in a cost effective manner. Furthermore, these metals can be of sufficient value to justify their recovery.
Metals have been removed from aqueous waste solutions utilizing processes such as ion exchange, electrolysis, and settling. However, all of these known processes have their limitations. Ion exchange is costly, slow and impractical. The ion exchange resins are expensive because they require complex and sophisticated fabrication processes. Some of this cost can be recouped by regenerating the ion exchange resins. However, the waste solutions produced during regeneration typically have to be treated. Similarly, electrolysis is also costly due to maintenance, resource requirements, and energy input. Electrolysis is also very sensitive to contaminants and generally provides ineffective levels of metal recovery.
Settling processes typically use one or more agents that transform the metals into materials that are no longer soluble in the system and settle to the bottom of the tank. However, currently known settling processes have the following limitations. Undesirably large amounts of sludge, which cannot be regenerated, can be formed. Some settling processes require heating to very high temperatures, e.g., greater than 80° C., to provide useful results. Still others require the use of a change in pH to cause the transformation of the metal into an insoluble material.
Accordingly, there is a need for effective recovery of metal ions from aqueous waste solutions (hereafter also referred to as aqueous process solutions).